# Kn2222A darlington pair + a relay, not functioning at low currents

As the title explains it, I have Kn2222A x2 transistors connected as a Darlington pair connected at the ground side of a [JQC-3F (T73) 5v] relay

The relay is connected to a 5v power supply.

the problem is, when I feed the base of the Darlington pair with low current, it won't switch the relay, it seems that the minimum current needed for the base is 0.3mA, less than that won't do a thing, looking at the data-sheet I should get at least 100*100 gain, 10000 * 0.3mA = 3A I am sure that's more than enough for the relay, unless I am missing something

I'm having hard time getting Hfe from the datasheet, and knowing the amp needed by the relay. Transistor datasheet

relay datasheet

another question is, can the relay function at higher than 5volts? (safely)

I'm only experiencing with electronics so any more info will be appreciated

I'm assuming your circuit looks like

simulate this circuit – Schematic created using CircuitLab

Assuming this is true, I think you're running up against a characteristic of the Darlington - high saturation voltage. In this configuration, the high gain does not carry over to the situation where you want to turn the transistor hard on. Instead of a transistor voltage of ~0.2 volts, something like 0.8 is more likely. This means that the relay is only getting about 4.2 volts, and this is close to the operating limit. As a result, you have to drive the Darlington harder than you think you need to.

simulate this circuit

This will allow more voltage across the relay.

In any event, compound transistor connections aren't magic. The problem is that at the very low current levels you expect with the first stage, current gain goes way down, so the overall gain is less than you expect from reading the data sheet.

Also note the catch diode. Always use one of these. If you don't, you'll get a turn-off voltage spike which may well kill your transistor.

A darlington transistor configuration cannot turn-on down to anything close to 0V. The first transistor will saturate (according to the data sheet at 0.3 volts) and, as it takes its "power" from the collector of the 2nd transistor, the 2nd transistor's collector will be probably at about 1V when fully activated. This means you are probably only getting 4V across the relay at best.

Hfe is only really guaranteed to be about 50 on the first transistor and possibly about 100 on the 2nd transistor. Read page 2 of the data sheet. But these are at voltages of 10V. Maybe this will be lower at 5V, I can't say for sure.

All this little factoids make me think your expectations of the transistor, when wired as a darlington, are too high. Maybe consider a MOSFET?

the gain is much lower at low currents . so the best you can do is 0.3mA. a darlington collector cannot go lower than 0.7 volts . so a 6 volt supply would be better.

• is it safe for the relay to use higher voltage? it says coil voltage on the datasheet 3-48v, does that mean i can use higher Vin without worrying about the relay?
– Ele
Commented May 8, 2015 at 22:06
• @ele: There are nine different relays described on the datasheet which differ only in the coil voltage. The coil voltage is given in the last digit or two of the part number, and may be 3, 5, 6, 9, 12, 18, 24, 36, or 48 volts. If the part number on your relay ends in "-5", it is designed for use with a 5 volt power supply. If you wanted to run it from 12 volts, you would need one with the part number ending in "-12". Commented May 8, 2015 at 23:02